Gmail's new Compose window docks the small message window in the bottom right corner of your inbox. If you'd rather move the window elsewhere or resize it for more space, you could click the popout arrow in the window, or, better yet, use this shortcut to quickly open the message window centred on your screen.
To make the window popout immediately, hold down Shift while clicking the Compose button, or hit Shift + C. You'll have more room to write and can work on multiple drafts at once.
This tip comes via the ProfHacker blog on the Chronicle of Higher Education. Writer Natalie Houston offers additional tips for getting used to the new Gmail Compose interface, such as clicking "To" in the message window to access your contacts list.
Getting Used to the New Gmail Compose [ProfHacker]